1. Field of the Invention
The present invention relates to a composite oxide, which is useful as a support for a catalyst for purifying an exhaust gas, a process for producing the same, a catalyst for purifying an exhaust gas, in which the composite oxide is employed as a support, and a process for producing the same.
2. Description of the Related Art
Conventionally, as a catalyst for purifying an automotive exhaust gas, a 3-way catalyst is used which oxidizes CO and HC and reduces NOx in the exhaust gas simultaneously. As for such a 3-way catalyst, for example, a catalyst has been known widely in which a support layer, being composed of γ-Al203, is formed on a heat resistant honeycomb substrate, being composed of cordierite, etc., and a catalyst ingredient, such as platinum (Pt), rhodium (Rh), etc., is loaded in the support layer.
By the way, as the conditions required for the support used in the catalyst for purifying an exhaust gas, a large specific surface area and a high heat resistance can be listed. In general, Al2O3, SiO2, ZrO2, TiO2, etc., have been used often. Further, by combinedly using CeO2 having an oxygen storage-and-lease ability, it has been carried out relieving the atmosphere fluctuation of exhaust gas. Furthermore, it has been known that the durability of the oxygen storage-and-release ability of CeO2 can be improved by making CeO2 into a composite oxide with ZrO2.
However, in the conventional catalyst for purifying an exhaust gas, there arise the decrement of the specific surface area of the support by sintering and the granular growth of the catalyst ingredient when it is subjected to a high temperature exceeding 800° C. Moreover, since the oxygen storage-and-release ability, possessed by CeO2, decreases as well, there has been a drawback in that the purifying performance of the conventional catalyst degrades sharply.
Since the exhaust gas emission control has been strengthened recently, it has been required strongly to purify an exhaust gas even in a very short period of time from starting an engine. In order to do so, it is required to activate the catalysts at a much lower temperature and to purify the emission-controlled components. Among them, a catalyst, in which Pt is loaded on CeO2, is excellent in terms of the performance for purifying CO starting at a low temperature. When such a catalyst is used, the CO-adsorption poisoning of Pt is relieved by igniting CO at a low temperature, and the igniting ability of HC is enhanced. Further, with these advantageous effects, the warm-up of the catalyst surface is facilitated, and accordingly it is possible to purify HC from a low temperature region. Furthermore, in this catalyst, H2 is produced by a water gas shift reaction in a low temperature region, and consequently it is possible to reduce and purify NOx by the reactions of H2 and NOx from a low temperature region.
However, the conventional catalyst, in which Pt, etc., is loaded on CeO2, lacks the durability in actual exhaust gases. It is not practical because CeO2 causes the sintering by heat. In order to use it in actual exhaust gases, it is necessary to upgrade the heat resistance without losing the properties of CeO2. Moreover, accompanied by the sintering of CeO2, Pt causes the granular growth so that there may arise a case in that the activity decreases. Hence, it has been required to stabilize Pt loaded on the support.
Even in a catalyst which includes CeO2 in its support, its oxygen storage-and-release ability, which is exhibited by CeO2, lowers when it is exposed to a high temperature. The disadvantage is caused by the sintering of CeO2, the granular growth of the noble metal loaded thereon, the oxidation of the noble metal, the solving of Rh in CeO2, and so on. Thus, in a catalyst which exhibits a low oxygen storage-and-release ability (or which has a small CeO2 content), the novel metal is likely to be exposed to a fluctuating atmosphere, and the deterioration (e.g., the agglomeration or solving) of the noble metal is furthermore facilitated.
Therefore, in Japanese Unexamined Patent Publication (KOKAI) No. 4-4,043, there is disclosed a catalyst for purifying an exhaust gas in which a catalytic ingredient is loaded on a composite oxide support being composed of a composite oxide of Al2O3, CeO2 and ZrO2. The catalyst, in which an arbitrary noble metal is loaded on such a composite oxide support, has high purifying performance even after it is subjected to a high temperature calcining at 850° C. The publication sets forth the reason for the advantage that the decrement of the oxygen storage-and-release ability is suppressed. Moreover, in Japanese Unexamined Patent Publication (KOKAI) No. 7-300,315, there is disclosed an oxide support, which is formed by precipitating Ce ions and Zr ions by adding charged particles (e.g., Al2O3).
Such composite oxide supports are produced in the following manner. Oxide precursors, being composed of a plurality of metallic elements, are prepared by an alkoxide method, a co-precipitation method, and the like, and are calcined thereafter. Among them, since the co-precipitation method is less expensive in terms of the material cost compared to that of the alkoxide method, it effects an advantage in that the resulting composite oxide is less expensive. Hence, the co-precipitation method has been used widely in the production of composite oxides.
For instance, in Japanese Unexamined Patent Publication (KOKAI) No. 9-141,098, there is set forth a catalyst for purifying an exhaust gas, which has Rh, serving as the catalytic ingredient, and a catalytic component loading layer, constituted by a composite oxide. The catalytic component loading layer is made in the following manner. Precipitates are co-precipitated from an aqueous mixture solution, being composed of a first water-soluble metallic salt including at least one metal selected from the group consisting of Mg, Ca, Sr, Ba, Y and La and a second water-soluble metallic salt including Zr. Then, the resulting precipitates are calcined to form the composite oxide, which constitutes the catalytic component loading layer. By thus making the composite oxide support, the high temperature durability and catalytic activity of Rh are upgraded, and the low temperature activity and purifying performance of the catalyst are improved remarkably even after a high temperature durability test.
Moreover, since Rh is good in terms of the reducing activity, it is one of the essential catalytic metal to a catalyst for purifying an exhaust gas along with Pt, which exhibits a high oxidizing activity. However, in the aforementioned catalyst, in which Pt and Rh are loaded on the composite oxide support, the granular growth of Pt can be controlled, but there still remains a drawback in that a solid phase reaction takes place between Rh and CeO2 in a high temperature oxidizing atmosphere so that Rh loses the activity.
While, in Japanese Patent Publication No. 2,893,648, a catalyst is reported which uses a support being composed of porous fine particles. The porous fine particles are a mixture of alumina and lanthanum oxide, which is produced by a co-precipitation method, and have pores, which have a pore diameter of 400 Å or less, in a percentage of from 20 to 30%. Since the porous fine particles are good in terms of the heat resistance, they can inhibit the granular growth of the noble metal. Moreover, there occurs no solid phase reaction between the porous fine particles and Rh.
In Japanese Patent Publication No. 253,516, etc., there is disclosed a catalyst for purifying an exhaust gas. In the catalyst, the coating layer is made into a two-layered construction, a catalytic layer with Pt loaded on Al2O3 is formed as a lower layer, and a catalytic layer with Rh loaded on a support, being composed of Al2O3 and ZrO2, is formed as an upper layer. By thus making a catalyst having such a plurality of catalytic layers, the functions of the respective layers can be shared more efficiently, and the activities can be enhanced. In addition, it is possible to control the drawbacks, such as the granular growth of the catalytic ingredient, caused by the mutual actions between the components, and so on.
While, a zeolite has an HC adsorbing ability. Hence, by using a zeolite for a support, HC are adsorbed onto the zeolite to control the emission in a low temperature range, and the HC, which are released from the zeolite, are oxidized in a high temperature region in which the catalytic ingredient is heated to the activation temperature or more. Therefore, it has been known to improve the conversion of HC from a low temperature to a high temperature by the operations. Accordingly, when a support is used in which a zeolite and CeO2 are used combindely, the oxygen storage-and-release ability is exhibited simultaneously in addition to the HC adsorbing ability. Thus, it is expected that the conversion of HC is furthermore upgraded by adjusting the atmosphere fluctuation.
Moreover, an NOx storage-and-reduction type catalyst has been recently put into an actual application as a catalyst for purifying an exhaust gas, which is emitted by a lean-burn gasoline engine. This NOx storage-and-reduction type catalyst is made by loading an NOx storage member, such as an alkaline metal, an alkaline-earth metal, etc., as well as a noble metal on a porous support, such as Al2O3, etc. In the operation of this NOx storage-and-reduction type catalyst, the air-fuel ratio is controlled from the fuel-lean side to the stoichiometric air-fuel ratio as well as the fuel-rich side in a pulsating manner. Hence, NOx are adsorbed onto the NOx member on the fuel-lean side. Then, the adsorbed NOx are released from the No, storage member at the stoichiometric air-fuel ratio and on the fuel-rich side, and are reduced and purified by reacting with the reducing components, such as HC and CO, by the catalytic action of the noble metal. Accordingly, since the emission of the NOx is controlled on the fuel-lean side as well, a high NOx purifying ability can be exhibited as a whole.
However, in the exhaust gas, SO2 is included which is generated by burning sulfur (S) contained in the fuel. The sulfur is oxidized by the noble metal to turn into SO3 in an oxygen-excess atmosphere. Then, the SO3 is easily turned into a sulfuric acid by water vapor contained in the fuel. The SO3 and sulfuric acid react with the NOx storage member to generate sulfites and sulfates. Thus, it has been apparent that the NOx storage member is poisoned to deteriorate by the sulfites and sulfates. This phenomenon is referred to as the sulfur poisoning. Moreover, since the porous support, such as Al2O3, etc., has a quality that it is likely to adsorb SOx thereonto, there has arisen a problem in that the aforementioned sulfur poisoning is facilitated. Then, when the NOx adsorbing member is thus turned into sulfites and sulfates, it cannot store NOx any more. As a result, the aforementioned catalyst might suffer from a drawback that the purifying performance lowers.
Therefore, it is possible to think of using an oxide, such as TiO2, etc., which exhibits a high acidity. Since TiO2 exhibits an acidity higher than that of Al2O3, it exhibits a low affinity with respect to SOx. As a result, it is possible to inhibit the NOx storage member from the sulfur poisoning.
By the way, by the recent strengthening of the exhaust gas emission control, the increasing opportunities of high speed driving, or the like, the temperature of the exhaust gas has become extremely high, and accordingly it has been required to furthermore improve the durability of the catalyst. Moreover, there arises another problem of the lowering purifying-ability phenomenon (the sulfur poisoning of the catalytic ingredient), which is caused by the SOx. Namely, the SOx, which are generated by burning the sulfur component in the fuel, are adsorbed onto the support so that they cover the catalytic ingredient to cause the drawback.
However, in the conventional catalyst for purifying an exhaust gas in which the composite oxide was made into the support, there are limits in terms of the heat resistance and sulfur-poisoning resistance. It is believed that the disadvantages result from the fact that the characteristics of the respective metallic oxides are not fully revealed.
For example, in the catalyst set forth in Japanese Unexamined Patent Publication (KOKAI) No. 4-4,043, not only CeO2 and ZrO2, but also Al2O3, which is a component being mainly responsible for the heat resistance, grow granularly considerably when the catalyst is used in a high temperature region of 1,000° C. or more for a long period of time. Accordingly, there arises a drawback in that the catalytic metal, which is loaded on the support, is also likely to grow granularly. There also arises another problem in that the durability is not improved as much as it is expected.
In addition, TiO2 is good in terms of the sulfur-poisoning resistance. However, it is short of the initial purifying activity when it is used independently. Therefore, it is possible to think of using a composite oxide, in which TiO2 is composited with Al2 O3. Acatalyst, in which such a composite oxide is made into a support, is good in terms of the sulfur-poisoning resistance, and has a high specific surface area. However, even in this composite oxide, there arises a drawback in that Al2O3, which is a component being responsible for the heat resistance, grows granularly considerably.